Method and plant for preheating particulate or pulverulent material

ABSTRACT

Described is a method as well as a plant for preheating particulate or pulverulent material such as cement raw meal or similar material in a cyclone preheater ( 1 ), comprising at least two cyclone stages, each comprising a riser duct ( 2   a   , 2   b   , 2   c   , 2   d ) and a cyclone ( 1   a   , 1   b   , 1   c   , 1   d ). The method is peculiar in that a portion of the material which is fed to at least one cyclone stage is introduced to the first part of the riser duct, viewed in the direction of travel of the exhaust gases, and is heated from a temperature of maximum 450° C. to a temperature of at least 550° C., and in that the remaining material which is fed to the same cyclone stage is introduced into the last part of the said riser duct. As a result, there will be a reduction in the amount of SO 2  which is discharged from the cement plant preheater as emission, without a simultaneous increase in energy consumption.

The present invention relates to a method for preheating particulate orpulverulent material such as cement raw meal or similar material in acyclone preheater, comprising at least two cyclone stages, eachcomprising a riser duct and a cyclone.

The invention also relates to a plant for carrying out the method.

In the cement industry it is customary practice to use a so-calledcyclone preheater for preheating the cement raw meal prior to its beingburned in a kiln into cement clinker which is subsequently cooled in aclinker cooler. Typically, a cyclone preheater comprising four to sixcyclone stages is used. The raw meal is introduced in the first cyclonestage and heated by direct contact with hot exhaust gases from the kilnaccording to the counter flow principle. Preheaters of this kind aregenerally known from the patent literature and one example is providedin EP 0 455 301.

The raw materials which are used for the cement-making process oftencontain sulphides, for example in the form of pyrites (FeS₂) whichduring the heating process in the preheater will react with oxygen toform SO₂ which is entrained in the exhaust gas stream discharged fromthe preheater. SO₂ is formed by partial oxidation of, for example, FeS₂mainly within the temperature range 300 to 550° C. In a traditionalcement-making plant comprising a preheater with five cyclone stages theformation of SO₂ of sulphide-containing raw materials will typicallyoccur in the second cyclone stage which in this context is defined ascomprising the discharge duct for exhaust gases from the third cycloneand the second cyclone in which the raw materials are typically heatedfrom a temperature between 300 and 350° C. to a temperature around 500°C.

From EP 1 200 176 is known a method by means of which calcined raw mealis introduced into the exhaust gases at a location immediately after,viewed in the direction of travel of the exhaust gases, SO₂ has beenformed. In principle, this known method performs satisfactorily, but itsmain disadvantage is that it involves relatively substantial capitalcosts for additional processing equipment and additional operatingexpenses, primarily for energy.

Further, from AT 390 249 is known a method as well as a plant by meansof which a portion or all of the raw meal is introduced into a zone witha higher temperature and hence enhanced bonding capability for SO₂, orwhere an adjustment is made of the temperature in the overlying areawith a lower temperature which is fed with SO₂-containing exhaust gasesby means of hot gas from a hotter area of the kiln system. Thedisadvantage of this known technology is that it will inevitably lead toan elevated temperature of the exhaust gases leaving the preheater,hence entailing increased energy consumption.

It is the object of the present invention to provide a method as well asa plant for preheating particulate or pulverulent material by means ofwhich the aforementioned disadvantages will be reduced.

This object is achieved by means of a method of the kind mentioned inthe introduction and being characterized in that a portion of thematerial which is fed to at least one cyclone stage is introduced to thefirst part of the riser duct, viewed in the direction of travel of theexhaust gases, and is heated from a temperature of maximum 450° C. to atemperature of at least 550° C., and in that the remaining materialwhich is fed to the same cyclone stage is introduced to the last part ofthe said riser duct.

As a result, there will be a reduction in the amount of SO₂ which isdischarged from the cement plant preheater as emission, without asimultaneous increase in energy consumption. This is due to the factthat by introducing only a portion of the material in the first sectionof the riser duct, a hot zone is provided with a sufficient heat surplusto allow the formed SO₂ to react with the CaO and CaCO₃ naturallyoccurring in the raw meal for forming, respectively, CaSO₄ and CaSO₃ aswell as CO₂ and the fact that the remaining material is then introducedso that the discharge temperature of the specific cyclone stage isreduced to a level equivalent to that applying if the preheater wereoperated in traditional manner. Studies conducted by the applicantfiling the present patent application have thus shown a significantincrease in the degree of absorption of SO₂ on CaO and CaCO₃ attemperatures above 550° C., and that essentially all of the SO₂ which isformed by oxidation of the sulphides in the raw materials can thereforebe absorbed by the raw materials CaO and CaCO₃ if the temperature of theexhaust gases/raw meal suspension is raised to a level of minimum 550°C. prior to separation of the exhaust gases and the raw materials in thesubsequent preheater cyclone.

The SO₂ formation as a function of the temperature depends to a greatextent upon the composition of the cement raw meal. In actual practice,analyses of the raw meal will constitute the basis for determining themost cost-efficient initial temperature of the raw meal which must beheated to at least 550° C. in one and the same process step within onesingle cyclone stage. The absorption degree or the ability of CaO andCaCO₃ to absorb SO₂ as a function of the time depends also on thetemperature. The retention time of the exhaust gases as well as the rawmeal in the specific process step will thus be the main determinant ofthe minimum temperature to which the raw meal must be heated. Typically,the optimum initial temperature will be within the range 300 and 450°C., whereas the temperature to which the raw meal must be heated in theprocess step will typically range between 550 and 700° C.

Generally, all the raw meal which is discharged from the precedingcyclone stage at a temperature of maximum 450° C. can be heated to atemperature of minimum 550° C. within a cyclone stage. In a typicalcyclone preheater comprising five cyclone stages, the temperature of theexhaust gases which flow from the third cyclone stage to the secondcyclone stage will be at a level around 700° C., and so it willtypically not contain the sufficient amount of energy for heating allthe raw meal from maximum 450° C. to at least 550° C. For this to beachieved, the exhaust gases from the kiln or another high-temperaturezone can be introduced to the specific cyclone stage, or it may beachieved on the basis of firing in the cyclone stage. However, aspreviously noted both solutions will increase the temperature of theexhaust gases leaving the preheater, thereby adversely affecting theheat economy.

Instead it is preferred that only a portion of the raw meal is subjectedto the heating from maximum 450° C. to minimum 550° C. in a singleprocess step. More specifically, it is preferred that the quantity ofraw meal which is subjected to the heating from maximum 450° C. tominimum 550° C. in a single process step is adapted in accordance withthe temperature and volume of the exhaust gases flowing from the thirdcyclone stage to the second cyclone stage. This may be achieved bysplitting the raw meal stream. In a first preferred embodiment of theinvention the raw meal which is discharged from the first cyclone can besplit into at least two sub-streams, of which one is directed in normalmanner to and introduced into the riser duct of the second cyclone stageabove the exhaust gas outlet in the third cyclone, whereas the secondstream is introduced into this riser duct at a location immediatelyahead of the gas inlet in the second cyclone.

In a second alternative embodiment, the raw meal which is fed to thecyclone preheater may be split into at least two sub-streams, of whichone is also preheated in normal manner in the first cyclone stage andsubsequently directed to and introduced into the riser duct of thesecond cyclone stage immediately above the exhaust gas outlet in thethird cyclone, whereas the second sub-stream is bypassed the firstcyclone stage and introduced into the riser duct of the second cyclonestage at a location immediately before the gas inlet in the secondcyclone. In this embodiment the heat consumption may be a little higheras compared with the preferred embodiment.

In the second cyclone stage both embodiments will provide a first zonewith a relatively high temperature in which SO₂ formation and absorptioncan take place, and a second zone in which the remaining part of the rawmeal can be preheated so that the temperature decreases to a normallevel. In this way it will be possible to remove a significant amount ofthe SO₂ which is formed as a result of the sulphide content in the rawmeal without increasing the temperature of the exhaust gases, and hencethe heat consumption. Embodiments and combinations other than thosedescribed above are conceivable and must be considered as being coveredby the present patent application.

As mentioned above, the retention time of the exhaust gases as well asthe raw meal at a given temperature in the specific process step will bea factor in determining the capability of the existing CaO and CaCO₃ toabsorb the SO₂ within this time span. In a traditionally configuredcyclone preheater, the retention time of the exhaust gases in forexample the second cyclone stage will be relatively short, often between0.5-1 second, whereas the retention time of the raw meal will usually besomewhat longer, often around 10 seconds on average. With the specificpurpose being to increase the retention time for the suspension of rawmeal and exhaust gases in the process step in which the raw meal isheated from maximum 450° C. to minimum 550° C., thereby ensuring asufficient good mixing for the desired chemical reactions to occur, theriser duct or the duct connecting the subsequent process step with thecyclone in the specific process step may be extended and formed, forexample, as a swan neck comprising an upwardly directed first section, abend and a downwardly directed second section which is connected to thecyclone of the process step. In a second embodiment, the diameter of theriser duct or the duct may be increased over at least a part of itsextent.

The plant for carrying out the method according to the invention is ofthe kind comprising a cyclone preheater with at least two stages, eachcomprising a riser duct and a cyclone and being characterized in that itcomprises means for heating a portion of the material from a temperatureof maximum 450° C. to a temperature of at least 550° C. in one and thesame process step within one cyclone stage.

Further characteristics of the plant according to the invention willappear from the subsequent detailed description, the patent claims andthe drawing.

The invention will now be described in further details with reference tothe drawing, being diagrammatical and where

FIG. 1 shows a first preferred embodiment of a plant according to theinvention,

FIG. 2 shows a second alternative embodiment of a plant according to theinvention,

FIG. 3 shows a detail of the embodiment shown in FIG. 1,

FIG. 4 shows a detail of the embodiment shown in FIG. 2, and

FIG. 5 shows an alternative embodiment of the detail shown in FIG. 3.

FIGS. 1 and 2 show two approximately identical examples of kiln plantsfor manufacturing cement clinker. Both kiln plants shown are of theILC-type, but the invention can also be used in connection with plantsof the SLC-type or any other plants being a combination of such plants.

Each of the shown plants comprises a cyclone preheater 1 with fourcyclones 1 a, 1 b, 1 c and 1 d, where 1 a is the first cyclone, 1 b isthe second cyclone, 1 c is the third or next-to-last cyclone and Id isthe fourth and last cyclone. The cyclones are connected in series andsupplied with gas/raw meal suspension via riser ducts or gas ducts 2 a,2 b, 2 c and 2 d. The plants thus comprise four cyclone stages where thefirst cyclone stage is made up of the riser duct 2 a and the cyclone 1a, the second cyclone stage is made up of the riser duct 2 b and thecyclone 1 b, the third cyclone stage is made up of the riser duct 2 cand the cyclone 1 c and the fourth cyclone stage is made up of the riserduct 2 d and the cyclone 1 d.

The plants also incorporate a calciner 3 which comprises an opening 9for introducing preheated raw meal from the last cyclone 1 d via itsmaterial outlet 6, and being connected with a separation cyclone 4, arotary kiln 5 and a clinker cooler 7. The plants also comprise a kilnriser duct 10 for directing kiln exhaust gases to the calciner 3, and aduct 11 for directing preheated air from the clinker cooler 7 to thecalciner 3. Raw meal from a not shown raw mill plant is routed to thepreheater 1 via a duct 13 and is preheated in the preheater incounterflow to the exhaust gases and is subsequently discharged from thepreheater in the cyclone 1 d and directed to the calciner 3 in which itundergoes calcination. From the bottom outlet of the separation cyclone4 the calcined raw meal is subsequently routed via a duct 8 to therotary kiln 5 in which it is burned into cement clinker which issubsequently cooled in the clinker cooler 7. The exhaust gases from therotary kiln 5 and the calciner 3 are drawn from the calciner 3 throughthe cyclone 4 and up through the preheater by means of a schematicallyshown fan 14.

According to the invention a portion of the raw meal which is directedto the riser duct 2 b of the second cyclone stage is heated from atemperature of maximum 450° C. to a temperature of minimum 550° C.,whereas the remaining material, is subsequently introduced into the lastpart of the said riser duct so that the amount of SO₂ which reacts withthe CaO and CaCO₃, occurring naturally in the raw meal for forming CaSO₄and CaSO₃, respectively, is increased, thereby reducing the amount ofSO₂, which is discharged from the preheater of the cement plant in theform of emission.

In actual practice it is preferred that the amount of raw meal which issubjected to the heating from maximum 450° C. to minimum 550° C. in aprocess step is adjusted in relation to the temperature and volume ofthe exhaust gases flowing from the third cyclone stage to the secondcyclone stage. This can be achieved by splitting the raw meal stream asapparent from the embodiments shown in FIGS. 1 and 2.

In the first preferred embodiment, shown in FIG. 1, the raw mealdischarged from the first cyclone 1 a is split into at least twosub-streams by means of a splitter gate 15 or a similar mechanism, ofwhich one sub-stream is directed in normal manner to and introduced intothe first part of the riser duct 2 b of the second cyclone stageimmediately above the exhaust gas outlet in the third cyclone 1 c via aduct 15 a, whereas the second sub-stream is introduced via a duct 15 binto the last part of the riser duct 2 b of the second cyclone stageimmediately ahead of the gas inlet in the second cyclone 1 b.

In the second alternative embodiment, shown in FIG. 2, the raw mealwhich is fed to the cyclone preheater 1 is split into at least twosub-streams by means of a splitter gate 16 or a similar mechanism, ofwhich one sub-stream is introduced in normal manner via a duct 16 a intoand preheated in the riser duct 2 a of the first cyclone stage, and thenvia the first cyclone 1 a directed to and introduced into the first partof the riser duct 2 b of the second cyclone stage immediately above theexhaust gas outlet in the third cyclone 1 c, whereas the secondsub-stream via a duct 16 b is bypassed around the first cyclone stage 2a, 1 a and introduced into the riser duct 2 b of the second cyclonestage immediately ahead of the gas inlet in the second cyclone 1 b.

By means of both the described embodiments according to the invention itwill be possible to achieve a first zone with a relatively hightemperature in the lower part of the riser duct 2 b, in which zone theSO₂ formation and absorption can take place, and another zone in whichthe remaining part of the raw meal is preheated so that the temperatureis reduced to a normal level.

At some existing kiln plants for manufacturing cement clinker the firstcyclone stage comprises two so-called twin cyclones. In such cases itwould be obvious to utilize the split of the raw meal, which takes placebetween the twin cyclones. Thus, the raw meal from one of the twincyclones may be directed to and introduced into the first part of theriser duct 2 b of the second cyclone stage immediately above the exhaustgas outlet in the third cyclone 1 c via a duct 15 a, whereas the rawmeal from the second twin cyclone may be introduced into the last partof the riser duct 2 b of the second cyclone stage immediately ahead ofthe gas inlet in the second cyclone 1 b. The second twin cyclone mayadvantageously be placed at a higher location, so that the raw meal fromthis cyclone may be introduced into the riser duct 2 b also at a higherlocation.

Embodiments and combinations other than those described above areconceivable and must be considered as being covered by the presentpatent application.

The FIGS. 3 and 4 show how the riser duct or the duct 2 b may forexample be configured as a swan neck comprising an upwardly directedfirst section, a bend and a downwardly directed second section which isconnected to the cyclone 1 b, with the purpose being to increase theretention time of the suspension of raw meal and exhaust gases in theriser duct 2 b of the second cyclone stage. Hence it will be possible tooptimize the retention time for the exhaust gases as well as the rawmeal in the hot zone with a view to achieving the desired chemicalprocesses. Typically, it is preferred that the riser duct 2 b isconfigured in such a way that the retention time is extended by a factorbetween 3 and 5.

FIG. 5 shows how the residence time in the high temperature SO₂reduction zone may be increased without increasing the total buildingheight of the preheater tower 1 significantly. In shown embodiment, theriser duct 2 b extends up, down, and up again. A portion of the materialfrom cyclone 1 a is introduced into the riser duct 2 b just aftercyclone 1 c whereas the remaining portion of the material from 1 a isintroduced after the U-bend part of 2 b. Some of the suspended materialin riser duct 2 b will unavoidable separate out in the bottom of theU-bend of riser duct 2 b. However, this material may simply beintroduced into riser duct 2 c as shown in the figure. Heat simulationshave revealed that the total energy consumption per mass of clinkerproduced will decrease because of this additional separation in riserduct 2 b.

The present invention is not limited to the shown embodiments which areused for illustration only, thus allowing for the configuration ofnumerous alternative embodiments combinations of the shown embodimentswhich lie within the framework of the present invention.

1. A method for preheating particulate or pulverulent material in acyclone preheater comprising at least two cyclone stages, each cyclonestage comprising an associated riser duct and a cyclone, the methodcomprising: feeding a first portion of the material to a first cyclonestage, wherein the material is introduced to a first part of a riserduct of the first cyclone stage in the direction of travel of theexhaust gases; heating the first portion of material from a temperatureof maximum 450° C. to a temperature of at least 550° C.; and feedingremaining material to a last part of the riser duct of the first cyclonestage.
 2. The method according to claim 1, wherein the first portion ofthe material is heated from a temperature between 300 and 450° C. to atemperature between 550 and 700° C., before the remaining material isintroduced into the last part of the riser duct.
 3. The method accordingto claim 1, wherein the material that is discharged from the firstcyclone stage is split into at least two sub-streams, of which a firstsub-stream via a duct is directed to and introduced into a riser ductimmediately above the exhaust gas outlet in a third cyclone stage and asecond sub-stream is introduced via a duct into a riser duct at alocation immediately ahead of a gas inlet in a second cyclone stage. 4.The method according to claim 1, wherein the material which is fed tothe cyclone preheater is split into at least two sub-streams, of whichone sub-stream via a duct is introduced into and preheated in a firstriser duct and then via the first cyclone is directed to and introducedinto a riser duct immediately above an exhaust gas outlet in a thirdcyclone stage, whereas a second sub-stream via a duct is bypassed aroundthe first cyclone stage and introduced into a second riser ductimmediately ahead of a gas inlet in a second cyclone stage.
 5. Themethod according to claim 1, wherein the material comprises cement rawmeal.
 6. A plant for preheating particulate or pulverulent materialcomprising: a cyclone preheater having at least two cyclone stages, eachcyclone stage comprising an associated riser duct and a cyclone, whereinthe cyclone preheater comprises means for heating a portion of thematerial from a temperature of maximum 450° C. to a temperature of atleast 550° C. within one cyclone stage.
 7. The plant according to claim6, wherein the means for heating comprises splitter gates.
 8. The plantaccording to claim 6, wherein a duct is configured as a swan neckcomprising an upwardly directed first section, a bend and a downwardlydirected second section which is connected to an associated cyclone. 9.The plant according to claim 6, characterized in that a duct isconfigured with increased diameter over at least a part of its extent.